Methods for using a detector to monitor and detect channel occupancy

ABSTRACT

Methods for using a detector to monitor and detect channel occupancy are disclosed. The detector resides on a station within a network using a framed format having a periodic time structure. When non-cooperative transmissions are detected by the network, the detector assesses the availability of a backup channel enabling migration of the network. The backup channel serves to allow the network to migrate transparently when the current channel becomes unavailable. The backup channel, however, could be occupied by another network that results in the migrating network interfering with the network already using the backup channel. Thus, the detector detects active transmission sources on the backup channel to determine whether the backup channel is occupied. Methods for using the detector include scheduling detection intervals asynchronously. The asynchronous detection uses offsets from a reference point within a frame.

This invention was made with Government support under ContractFA8750-05-C-0150 awarded by the Air Force. The Government has certainrights in this invention.

FIELD OF THE INVENTION

The present invention relates to monitoring channels to avoidinterference with other wireless devices operating over the samechannels. More particularly, the present invention relates to differentmethods to detect occupancy of the channel designated as a backupchannel for wireless transmission sources prior to migration of thesources to the backup channel when the current channel is unavailable.

DESCRIPTION OF THE RELATED ART

Wireless networks enable connectivity between different stations, nodesand the like. The different stations may reside in different locationsand operate on frequency channel(s) designated for the network. Thenumber of channel allocations available depends on the total amount ofdesignated spectrum as well as spectrum occupancy.

Some networks are allowed to operate in any channel within thedesignated frequency spectrums as long as the channel is not being used.Channels occupied by the transmission sources already operating withinthe designated spectrum range are to be identified and avoided. Thetransmissions or signals from these sources may be referred to asnon-cooperative transmissions. Other forms of potential interference mayarise after the network is established. The network should vacate itschannel shortly upon detecting the presence of a non-cooperativetransmitter in order to avoid interference. Further, the migration ofthe network to a new channel should be transparent and seamless suchthat communications are not impacted.

One solution avoids interference with non-cooperative transmissions byshutting down the network until the frequency channel becomes available.This solution, however, is not feasible as the channel may never becomeavailable and the network cannot be offline for any period of time. Thenetwork also can transition from the current channel to anotherdesignated channel, or backup, channel, unless the backup channel isoccupied. Other networks may be using the frequency of the backupchannel, and any migration to the frequency would cause problems. Thus,the occupancy of both frequency channels impacts the sustainability ofthe network.

SUMMARY OF THE INVENTION

Thus, the present invention overcomes the problems within the artdiscussed above by implementing methods for using a detector to monitorand detect channel occupancy. Preferably, a network operates within achannel at a designated frequency. The network is assigned a backupchannel at a different frequency. Upon the detection of non-cooperativecommunications within the network, the stations within the networkvacate the original frequency channel and seamlessly transitionoperations to the backup channel.

The present invention also seeks to avoid interference withtransmissions or other networks within the backup frequency channel. Ifthe backup channel is occupied, then the current network should notmigrate to the backup channel to avoid interference with another networkor transmission source. The present invention discloses processes andmethods to reliably determine occupancy of the backup channel. Forexample, a detector uses detection intervals to determine whethernon-cooperative transmissions exist on the backup channel.

Non-cooperative transmissions may be detected during transmission gapsin the network transmissions. The gaps may be part of a frame structurewhen the network operates in a framed format. In a framed format,transmission gaps occur repeatedly; the detector is engaged by eachactive network station and the spectrum measurement is performed duringthe gap time intervals to monitor spectrum for the presence ofnon-cooperative signals in its vicinity in the current, or primary,channel and in the backup channel(s).

If a non-cooperative transmission is detected in the primary channel byany participating network station, the network initiates the migrationto the backup channel to avoid interference with a detectednon-cooperative source. Prior to switching channels, each networkstation verifies the availability of the backup channel. Successfulvalidation provides increased confidence that the migration to thebackup channel will not cause interference.

The proposed verification process involves asynchronous detection suchthat the spectrum measurement is not performed during the regulartransmission gap intervals, or any other periodically scheduled timeintervals, within the frame. The verification process is necessary toavoid the situation when the gaps used by the network stations toperform detection measurement are synchronized to the gaps ofnon-cooperative transmission source(s) operating over the intendedbackup channel. The situation is common when both channels are occupiedby the same type of network. The present invention avoids interferencedue to the possibility of synchronized transmission gaps.

According to embodiments of the present invention, a method for using adetector is disclosed. The method includes detecting non-cooperativetransmissions on a current, or primary, channel. Frame-based periodicdetection regions are used to monitor current channel(s) fornon-cooperative transmissions. The method also includes scheduling anasynchronous detection interval having a reference within a frame. Themethod also includes detecting presence of transmissions on a backupchannel with the detector during the detection interval.

According to further embodiments of the present invention, anothermethod for using a detector also is disclosed. The method includesdetecting interference at a station participating in a network. Thenetwork uses a current channel at a first frequency. The method alsoincludes scheduling a start of a detection interval for the detectorwithin a frame over the frequency channel. The method also includesdetecting channel occupancy on a backup channel using the detectorduring the scheduled detection interval. The backup channel uses asecond frequency different from the first frequency of the currentchannel.

According to further embodiments of the present invention, anothermethod for engaging a detector is disclosed. The method includesdetecting non-cooperative transmissions on a current channel. The methodalso includes scheduling a start of a detection interval in a frameusing an offset from a reference point within the frame. The offset isselected from a plurality of precomputed offsets. The method alsoincludes detecting non-cooperative transmissions on a backup channelduring the detection interval using the detector.

According to further embodiments of the present invention, anothermethod for engaging a detector is disclosed. The method includesdetecting non-cooperative transmissions on a current channel. The methodalso includes scheduling a start of a detection interval using a randomoffset within a frame. The method also includes detectingnon-cooperative transmissions on a backup channel using the detectorduring the detection interval.

According to further embodiments of the present invention, a method forusing a detector to determine availability of a backup channel isdisclosed. The act of performing detection for the backup channelinvolves abandonment of the current channel by the network station andtuning its frequency synthesizer frequency to enable the detector windowto overlap the backup channel in the frequency domain for the durationof a detection interval.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide further understandingof the invention and constitute a part of the specification. The figureslisted below illustrate embodiments of the invention and, together withthe description, serve to explain the principles of the invention.

FIG. 1A illustrates a network including stations according to thedisclosed embodiments.

FIG. 1B illustrates a representative frame used by a network accordingto the disclosed embodiments.

FIG. 2A illustrates a graphical representation of a migration from achannel to a backup channel according to the disclosed embodiments.

FIG. 2B illustrates time-frequency transmission graphs according to thedisclosed embodiments.

FIG. 3 illustrates a process for the detection of non-cooperativetransmissions on a backup channel according to the disclosedembodiments.

FIG. 4 illustrates a flowchart for monitoring and detecting channeloccupancy according to the disclosed embodiments.

FIG. 5 depicts a flowchart for detecting channel occupancy according tothe disclosed embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention. Examples of the preferred embodiments are illustratedin the accompanying drawings.

FIG. 1A depicts a network 202 including stations according to thedisclosed embodiments. Network 202 allows communication and dataexchange between the stations. Network 202 is not limited by geography,frequency spectrum or configuration. Network 202 must avoid interferingwith other networks or transmitters and operates on a frequency not inuse by other transmission sources. Once any interference is detected,network 202 switches from the present operating frequency to another.

Network 202 includes base station 52 and subscriber stations 54, 56, 58,60, 62, 64 and 66. The number of stations within network 202 is shownfor illustrative purposes only. The number of stations within network202 is not limited to the amount shown in FIG. 1, and may include anynumber of stations. Further, network 202 is not limited to theconfiguration shown in FIG. 1, which may be referred to as a hub andspoke configuration. Network 202 may be in any configuration known inthe art, and may not necessarily include a base station.

Base station 52 manages network 202 as needed, and may broadcast networkinformation to all the stations. Base station 52 also relayscommunications between the various subscriber stations. Base station 52determines when a switchover to a different operating frequency, orfrequency channel, is performed based on collective spectrum statusinformation from the participating subscriber stations.

Subscriber stations 54-66 within network 202 may communicate with basestation 52 and each other. Subscriber stations 54-66 are located atvarious distances and directions from base station 52. For example,subscriber station 58 may be located a distance 88 from base station 52.Subscriber station 62 is located a distance 90 from base station 52.Distance 90 is greater in value than distance 88. Other stations arelocated at different distances from base station 52.

Subscriber stations 54-66 may query base station 52 for various reasons.For example, subscriber stations 54-66 may query base station 52 whetherresources are available in network 202.

Each subscriber station is allotted time slots within the frame portionused for to transmit to base station 52. The size of allotted time slotmay not be the same for each subscriber station. For example, subscriberstation 66 may receive more time in a frame to transmit than subscriberstation 60. Base station 52 may allocate the time to the individualsubscriber stations according to policies or other considerations.Alternatively, all subscriber stations may receive the same transmittime within a frame.

Subscriber stations 54-66 and base station 52 also include detectorsthat monitor the occupancy of spectrum available to support networkoperations. FIG. 1A depicts a detector on each station, but any numberof detectors may be located at the stations. Base station 52 includesdetector 86. Subscriber stations 54, 56, 58, 60, 62, 64 and 66 includedetectors 72, 74, 76, 78, 80, 82 and 84, respectively. Detectors 72-86need not be identical or operate the same. Detectors 72-86, however, mayor may not have sufficient bandwidth, also referred to as effectivedetector bandwidth or a detector frequency window, to cover the entirespectrum range available to support network operations. Further,detectors 72-86 may be engaged (triggered) to perform detectionmeasurements over detection intervals to detect potentialnon-cooperative transmission sources.

FIG. 1A also depicts non-cooperative (NC) transmitter 68 andnon-cooperative (NC) radio 70, which are not considered part of network202. Network 202 may interfere with transmitter 68 and NC radio 70 whenthe stations transmit and receive communications that on the frequencychannel used by network 202.

Transmitter 68 may be a television or radio station that transmitssignals during certain hours of the day at a specified frequency. Thetransmission power of transmitter 68 may be large enough to interferewith all stations of network 202, or just enough to interfere with, forexample, subscriber stations 54 and 56. In any event, detectors 72 and74 at stations 54 and 56 will alert base station 52 that anon-cooperative transmission source is detected at a frequency regionused by transmitter 68 so that appropriate action may be taken. One suchaction is switching network 202 over to a different frequency

NC radio 70 is an authorized radio transmission source that is not amember of network 202. NC radio 70 may be a member of a differentnetwork. NC radio 70 may enjoy long periods of time without transmittingany information, and during these periods is transparent to network 202.At certain times, however, NC radio 70 transmits signals with enoughpower so that it can be detected by members of network 202. If NC radio70 uses the same channel as network 202, then upon the detection ofnon-cooperative transmissions originating from NC radio 70, network 202must vacate the current frequency channel to avoid interfering with NCradio 70.

FIG. 1B depicts a time frame format 100 for used by network, such asnetwork 202, according to the disclosed embodiments. Preferably, a framerefers to a periodic structure subdivided into time slots assigned tothe stations of network 202 for data transmissions. Certain time slotsmay be not used for data communications, but support other functionalitysuch as acquisition and tracking or detection.

Referring back to FIG. 1A as an example, network 202 includes a basestation 52 and at least one subscriber station. Essentially, frame 100allows stations to receive and send data at specified intervals withinthe frame. All stations within network 202, for example, are given theopportunity to transmit information within frame 100.

Uplink sub-frame 103 of frame 100 is comprised of the control slots 102and communication slots 104 used by base station 52 to transmitinformation to subscriber stations participating in network 202. Basestation 52 uses control slots 102 in every frame to transmit requiredcontrol information. Base station 52 may use all, some or none ofcommunication slots 104 to send information to subscriber stations 54-66of network 202. During uplink sub-frame 103, subscriber stations 54-66do not transmit and are set to receive information from base station 52.

Sub-frame 106 may be referred to as the downlink sub-frame because thisportion of frame 100 used by subscriber stations 54-66 to transmitinformation to base station 52. During sub-frame 106, base station 52does not transmit but receives information from the participatingsubscriber stations. Each subscriber station is allocated a portion ofsub-frame 106 to transmit.

For example, as depicted in FIG. 1B, sub-frame 106 may be broken intosubscriber portions 114, 116, 118 and 120. Subscriber portion 114corresponds to one subscriber station, such as station 54 in network202. Subscriber portion 116 corresponds to station 56, subscriberportion 118 corresponds to station 58, and so on. Subscriber portion 120represents the remaining portions for the rest of the stations, and maybe any number. The subscriber portions are not necessarily equal anddifferent stations may have larger time allocations to transmit to basestation 52.

While each subscriber station is transmitting, the other stations do nottransmit and wait until their allocated portion to start transmission.Thus, only one station at time preferably is transmitting to preventinterference between the transmitting station and other stations withinnetwork 202. Base station 52 receives communications from theappropriately scheduled station. For example, subscriber portion 114corresponds to station 54. During this period, station 54 transmitsinformation to base station 52 and stations 56-66 should not betransmitting. Thus, subscriber stations should transmit during theirassigned subscriber portions. At the end of subscriber portion 114,station 54 ceases transmission and subscriber portion 116 starts withstation 56 transmitting information.

Other features of frame 100 include transmission gaps 108 and 110. A gaprefers to the portion of frame 100 not used for transmission by anynetwork station. Gap 108 is located between sub-frame 104 and sub-frame106. Gap 110 is located at between sub-frame 106 and end 112 of frame100.

FIG. 2A depicts graphical representations of switching from a networkfrequency to a backup channel according to the disclosed embodiments.Bar graph 210 shows current, or primary, channel 211 at frequency f1used by network 202, backup channel 206 at frequency fb used by network202 and primary channel 213 at frequency f2 used by a different network208.

Referring to bar graph 210, a channel at frequency fb is used by network202 as a backup channel. A backup channel refers to an unoccupiedchannel at another frequency that is chosen by network 202 for migrationto if the current channel network 202 operates on becomes unavailable.Network 202 may migrate from channel 211 to a designated backup channel206 when a non-cooperative signal is detected by any stationparticipating in network 202 operating on channel 211. Network 202 thencontinues its operations using new primary channel 206 and a channel atanother frequency is designated as a backup channel. Therefore, network202 does not interfere with any other network or transmitter whileoperational.

Network 208 operates in a channel 213 at frequency f2 different from thefrequencies f1 and fb used by network 202. As shown in bar graph 210, nointerference between networks 202 and 208 takes place.

Bar graph 212, however, shows a different scenario when network 208 isoperating in a channel designated as a backup channel for network 202.After non-cooperative transmissions are detected by network 202 inchannel 211, the migration to backup frequency f2 will causeinterference with network 208.

FIG. 2B depicts a time-frequency map according to the disclosedembodiments. The bar graphs depict frequency channels for a currentnetwork, such as network 202, and a second network, such as network 208.FIG. 2B shows the frame allocations used by these networks and thenotional power level of the transmissions within the frames, such asframe 100 shown in FIG. 1B. Further, FIG. 2B shows detection timeintervals used by detectors at a base station and at least onesubscriber station.

Bar graph 220 represents the frames and notional power levels in thechannel at frequency f1 occupied by network 202. Bar graph 220 shows thedownlink and the uplink sub-frames for frames, such as frame 100. Forexample, frame 100 is shown on a time axis for bar graph 220. Frame 100includes uplink sub-frame 104, downlink sub-frame 106 and gaps 108 and110. The pattern of frame 100 is repeated over time within network 202.

When non-cooperative transmissions are detected in a channel used bynetwork 202, network 202 migrates to a backup channel withoutinterruption of network operation. Network 202, however, should confirmthe absence of any non-cooperative transmissions on the backup channel.If the backup channel is unoccupied, the migration proceeds. If,however, network 208 occupies the backup channel as depicted by bargraph 228, the migration does not take place, thereby preventing network202 from interfering with network 208.

A detection interval refers to an interval of time within frame 100 thatis used by the detector to perform a detection measurement covering afrequency range contained within the detector window. For example, thenon-cooperative transmissions may be detected if the signals or energyfrom a non-cooperative transmission source is detected in the detectionwindow during the detection interval. Following the example, a detectorat a station in network 202 detects signals from a transmission sourceof network 208 at frequency f2, during a detection interval scheduledwithin gap 108.

If transmission gaps of network 202 are used as detection intervals anddo not time overlap with the transmission gaps of network 208, thenlittle or no probability exists that that transmission of network 208will remain undetected by network 202. Alternatively, if thetransmission gaps of network 202 used as detection periods time overlap,or are synchronized, with the transmission gaps of network 208, thennetwork 208 operating at frequency f2 will remain undetected by network202. Any migration to frequency f2 results in interference with network208.

The synchronization issue is overcome by implementing asynchronousdetection. Detection intervals are scheduled asynchronously to avoid any“blind spots” on the backup channel. Blind spots may be time periodswhen the non-cooperative transmissions are unobservable to the detector.The detector can control what time intervals used for detection. Byshifting detection intervals away from the transmission gaps, network202 avoids missing detection of network 208 due to gap synchronization.

Examples of detection intervals, also referred to as detection periods,may be seen in bar graphs 222 and 224. Detection measurement isperformed over a time interval within the frame. The term triggering mayrefer to starting the detection measurement at the discrete point inframe 100. The detection interval may last as long as desired or neededto make an accurate measurement.

Bar graph 222 shows detection intervals 233 of a station on network 202.Detection intervals 233 allow a detector at the station of network 202to detect network 208 transmissions on the backup channel when gap 108in frame 100 is not synchronized in time with gaps in frame 238.Detection periods 233, however, do not allow a detector at the stationof network 202 to detect network 208 transmissions on the backup channelwhen gap 108 in frame 100 is synchronized in time with gaps in frame240, as shown by dotted line in 228. Thus, network 208 operates over achannel at frequency f2 and is undetectable by network 202.

By changing detector interval starting points, detection intervals usedby a station in network 202 are shifted from the transmission gaps offrame 100 to enable asynchronous detection. Therefore, the detectionintervals do not overlap in time with the transmission gaps of frame 240used by network 208

FIG. 2B also depicts frequency f1 of network 202 and frequency f2 ofnetwork 208 within a detector window 250 of the detector, such asdetector 72 on subscriber station 54 of FIG. 1A. A detector is able tosimultaneously monitor both frequencies only if they fall within thedetector window. The station tunes its frequency so that the detectorcan monitor the channel at frequency f2 for the duration of thedetection interval if the frequencies are unobservable in the samedetector window used to monitor frequency f1. Thus, the presentinvention may detect transmissions at frequency channels spanning morethan a single detection window.

One restraint on the scheduling of an offset is the region, or portion,of the frame acceptable for the detection interval. These regions may bereferred to as eligible detection intervals. One reason for therestraint is that a station cannot transmit and detect at the same time.For example, the detector may not perform detection operations while thestation is transmitting during its corresponding sub-frame. Thus, theeligible detection intervals may represent portions of the frame thatthe station does not transmit and during which the detector may schedulea detection interval.

FIG. 3 depicts a process for the detecting during detection intervals300 and 310 by a detector according to the disclosed embodiments. Theprocess of FIG. 3 seeks to avoid the problem of the detector usingdetection intervals 230 and 233 when synchronized with the frame of thenetwork 208, as shown above. The detector would not detect anything asno transmission or activity is observable on the backup channel duringthe detection intervals. In this case, a switch to the backup channelwill result in non-cooperative interference to network 208.

FIG. 3 shows a process using offsets to schedule detection periodsintervals within frame 100. Thus, the detector will detect interferenceon the backup channel before migration. Preferably, multiple frames,such as frame 100, are used to monitor and detect transmissions on thebackup channel. For example, three (3) frames may be used. Morepreferably, the offsets are varied in order to cover a wider range ofdetection intervals that increase reliability of detection of thenon-cooperative transmissions before migration.

For example, the base station, such as base station 52, is configured toenable detection intervals 300. Offsets 302, 304 and 306 are used todetermine when to schedule the start of detection intervals 300. Thus,detection intervals 300 are offset from reference points 320 within theframe. Reference points 320 may occur periodically with the frame.Offsets 302, 304 and 306 vary in value so as to stagger detectionintervals 300. Thus, the detector schedules detection intervals 300asynchronously. The detector avoids a repetitive pattern for detectionby having different values for the offsets.

For example, offset 306 may be the smallest offset value to positiondetection intervals 300. Offset 304 may be the largest offset value toposition detection intervals 230 further away from its reference point320. Offset 302 may include a value between offsets 306 and 304 to placeits detection interval 300 about midway between reference points 230.Thus, detection intervals 230 are scheduled at three different locationswithin frame 100. With this coverage, the detector provides resultshaving more confidence that non-cooperative transmissions are notpresent on the backup channel than then the disclosed synchronousdetection routine.

Detection intervals 233 at the station, such as subscriber station 54,are offset in a similar manner. Offsets 312, 314 and 316 are used toposition detection intervals 310 from reference points 324. For example,offset 312 may have a small value so that its detection interval 310 isnot far away from reference point 324. Offset 314 may have the largestoffset value and offset 316 may place a detection interval 310 halfwaybetween reference points 324. Thus, all of detection intervals 310 arescheduled at three different locations in frame 100.

If detection intervals 320 and 324 are staggered by the various offsets,then the detection intervals cover much of frame 100. Preferably, thedetector does multiple detection measurements using the offsets afternon-cooperative transmissions are detected on network 202. As notedabove, the preferred number of detection measurements is three (3).

In addition to the embodiments using a set offset, as disclosed above,other embodiments of the present invention may use random offsets toschedule the detection intervals. Thus, a detection interval isscheduled arbitrarily. The trigger point for each detection measurementis random.

When the primary and backup frequencies are spread beyond thespecifications for the detector window, the station uses itstransmission allocation in frame 100 to tune its frequency to coverbackup channel. The station ceases transmission during its allocatedperiod, buffers the information to be transmitted, tunes its frequencyto cover the backup channel in a detection window, uses one of theabove-disclosed deterministic or random offset asynchronous detectionprocesses to determine non-cooperative transmissions in the backupchannel during its normal transmission period, performs detectionmeasurement, and then tunes its frequency back to the primary frequency.Instead of transmitting information, the station stores the informationfor transmission later, after the detection is completed.

FIG. 4 is a flowchart for monitoring and detecting channel occupancyaccording to the disclosed embodiments. The flowchart shows a processfor detecting non-cooperative transmissions on a backup channel. Step502 executes by detecting the non-cooperative transmissions within thenetwork on the current, or primary, channel, such as network 202disclosed above. The detecting station may alert the other stationswithin network 202.

Step 504 executes by enabling an asynchronous detection method. Anasynchronous detection method refers to the deterministic or random timeoffset in scheduling of the detection intervals allowing the detector todetect transmissions on the backup channel. Base station 52 andsubscriber stations 54-66, may suspend their typical frame uplink anddownlink routine in order to detect potential non-cooperativetransmissions on the backup channel for network 202, such as backupchannel 206. All the stations actively participating in network 202 mustclear the backup channel.

Step 506 executes by performing the detection on the backup channel.This step is disclosed in greater detail below in FIG. 5. A synchronousdetection using detection intervals during gaps in the frames, such asgaps 108 and 110 disclosed above may be executed in addition to themethods using asynchronous detection.

Step 508 executes by processing the detection information obtainedduring asynchronous and synchronous detections. If no transmissionsources are detected during the detection interval(s), then the backupchannel is declared clear of a non-cooperative transmitter or network,such as network 208. If transmissions sources are detected, then backupchannel is declared as unavailable.

Step 510 executes by determining the detection decision. If the decisionindicates that “the backup channel is clear”, then step 512 executes byproceeding with migration of network 202 to the backup channel at abackup frequency. After migration, a new backup channel on a new backupfrequency is selected. If the decision indicates that “the backupchannel is not clear”, step 514 executes and no migration takes place.

FIG. 5 depicts a flowchart for detecting channel occupancy according tothe disclosed embodiments. FIG. 5 corresponds to step 506 in FIG. 4.FIG. 4, however, is not limited by the embodiments disclosed by FIG. 5.Further, FIG. 5 is not limited by FIG. 4. FIG. 5 discloses some of thedifferent methods for determining whether a frequency channel isoccupied.

Step 602 executes by receiving an instruction from the applicablestation to perform detection on a backup channel because the stationdetected non-cooperative transmissions or directed to do so by thenetwork control entity such as, for example, the base station. Forexample, a station, such as station 54, detects interference on currentfrequency f1 and instructs its detector to determine channel occupancyof the backup channel. Alternatively, the instruction is received from abase station or other station in network 202 to perform the detection.

Step 604 executes by determining whether the detection method desiresthe use of a deterministic or random offset in scheduling of thedetection intervals. A deterministic offset refers to a pre-scheduledsetting of the start time of the detection intervals during theasynchronous detection. The detector or station may want todeterministically control when the detection intervals occur. Randomoffset refers to random setting of the start time of the detectionintervals during asynchronous detection.

If step 604 is yes, then step 606 executes by retrieving the offsetsused to schedule the start of the detection periods. An offset isselected from a set of offsets. Each offset may have a different “value”in that they are do not provide the same offset for the detectionintervals. Referring to FIG. 3, offsets 302, 304 and 306 determine thedetection intervals for a base station and offsets 312, 314 and 316determine the detection intervals for a detector on a subscriberstation.

The offsets may be set according to a desired pattern to detect over asmany different points in a frame as possible. The offsets may be set bysoftware on the station, but may not have the same value. In otherwords, the offsets vary the amount “offset” from a reference pointwithin the frame.

If step 604 is no, then random scheduling of the detection intervals isto be used. Step 608 executes by randomly determining offsets for thestart of the detection intervals. Thus, the detection intervals arerandomly positioned to allow the detector to detect any non-cooperativetransmissions or signals on the backup channel. The random offsets arepositioned from the reference points within the frame.

Step 610 executes by determining whether the backup channel for thecurrent network is within the same detector frequency window as theprimary channel. For example, detector 74 at subscriber station 56should determine whether backup frequency fb is detectable in thedetector frequency window when tuned to operate at frequency f1. If not,then the detector suspends transmissions and tunes the station toinclude backup frequency fb in the detector frequency window, asdisclosed below.

If step 610 is yes, then step 612 executes by scheduling the detectionintervals using the offsets. Step 612 schedules the detection intervalsby offsetting their start from at least one reference point within theframe(s). The regions allowable for detection intervals depend on a fewvariables, such as type of station, transmit or receive status, and thelike.

If step 610 is no, then step 614 executes by scheduling the detectionintervals using the offsets in the transmission sub-frame of thestation. Step 614 schedules the detection intervals by offsetting theirstart from at least one reference point within the frame(s). The regionsallowable for detection intervals depend on a few variables, such astype of station, transmit or receive status, and the like.

Step 616 executes by buffering the data normally scheduled to betransmitted during the applicable frame interval(s) when they are notused as detection interval(s). If the station is one or more ofsubscriber stations 54-66, for example, then the data scheduled fortransmission during the downlink, or sub-frame 106, portion is buffered.A subsequent frame(s) transmits the buffered data during its normaldownlink or uplink time.

Step 618 executes by suspending transmit operations for the appropriateportion of frame 100. As with the buffered data, the transmission at thestation is suspended so that the station can tune onto a frequency toencompass the backup channel and perform a detection measurement. Forexample, base station 52 includes detector 86. Base station 52 transmitsduring sub-frame 104 in normal operations. In step 618, however,transmission is suspended during sub-frame 104 which enables detectionoperations on the backup channel. Step 620 executes by tuning thestation to frequency so that the detector window encompasses thefrequency fb of the backup channel.

Step 624 executes by detecting the presence of non-cooperativetransmissions, or signals, on the backup channel using the detectorduring the detection interval. The detection intervals occur and allowthe detector to detect, or measure, any non-cooperative transmissions onthe backup frequency channel to determine if the channel is occupied.

Step 632 executes by tuning the station back to the current ifapplicable. If steps 620-624 were not executed, then step 630 isskipped. Step 630 executes by processing the detection data taken by thedetector.

Step 634 executes by storing the detection results for the detectionintervals. The stored data may be used for processing and determiningwhether the backup channel is occupied. Step 636 executes by determiningwhether to repeat the process of scheduling a detection interval anddetecting transmissions on the backup channel. As disclosed above, thedetector may desire multiple sets of measurements for a more confidentestimation of channel occupancy. Preferably, three sets of measurementsare taken before determining whether to migrate. Thus, on the third timethrough steps 612-632, step 636 may be “NO” to indicate the end of theprocess.

Step 638 executes by returning control of the flowchart back to themigration process of the station. The stored information on what wasdetected on the backup channel during the detection intervals may beused to determine whether the backup channel is occupied by anothernetwork or transmitter that would be interfered with by the currentnetwork.

Thus, the present invention discloses various methods and process formonitoring channels and frequencies. The station of the network detectsnon-cooperative transmission on its current frequency. Before switchingthe network over to the backup frequency, the present invention performsdetections on the backup channel to verify if it is not occupied. Usingan occupied channel will causes interference with the wireless devicesactively operating at that channel. The present invention may use one ofthe above-disclosed methods to detect non-cooperative transmissionsources active on the backup channel.

The principles and scope of the present invention will be apparent tothose skilled in the art. Further, various modifications and variationscan be made in the disclosed embodiments without departing from thespirit of the invention. Thus, the present invention covers themodifications and variations of the preferred embodiments disclosedabove provided that they come within the scope of any of the claims ortheir equivalents.

The invention claimed is:
 1. A method comprising, in a cooperativenetwork of radio frequency communication devices, detecting anon-cooperative transmission on a first radio frequency channel;scheduling a detection interval in a plurality of frames among aplurality of devices in the network, wherein each detection interval isscheduled to begin at a different offset within the frame relative tooffsets at which each other detection interval is scheduled.
 2. Themethod of claim 1, wherein the step of detecting the non-cooperativetransmission is performed over a plurality of time intervals.
 3. Themethod of claim 2, wherein the plurality of time intervals occurs over aplurality of transmission frames of the cooperative network.
 4. Themethod of claim 2, wherein the plurality of time intervals occurs over aplurality of transmission frames of the non-cooperative transmission. 5.The method of claim 1, wherein the detection interval in each of theplurality of frames spans a different length of time.
 6. A systemcomprising: a plurality of radio frequency communication devicesconfigured to communicate on a first channel; a plurality of radiofrequency detectors, each of the plurality of radio frequency detectorsconfigured to schedule a detection interval in each of a plurality offrames, wherein each detection interval is scheduled to begin at adifferent offset within the frame relative to offsets at which eachother detection interval is scheduled.
 7. The system of claim 6, whereinthe plurality of radio frequency detectors comprises non-homogeneousdetectors.
 8. The system of claim 6, wherein the plurality of devicescomprises a base station, and wherein at least one of the plurality ofdetectors is configured to inform the base station of a detectednon-cooperative signal on the first channel.
 9. The system of claim 6,wherein the detectors are configured to detect a non-cooperativetransmission on the first channel.
 10. The system of claim 9, whereinthe detectors are configured to detect the non-cooperative transmissionover a plurality of time intervals.
 11. The system of claim 10, whereinthe plurality of time intervals occurs over a plurality of transmissionframes of the cooperative network.
 12. The system of claim 11, whereinthe plurality of time intervals occurs over a plurality of transmissionframes of the non-cooperative transmission.
 13. The system of claim 6,wherein the detection interval in each of the plurality of frames spansa different length of time.
 14. The system of claim 6, wherein theplurality of devices includes a base station and a subscriber station.15. The system of claim 14, wherein the detection intervals are offsetat different amounts in the frames within which the detection intervalsare scheduled for the base station and for the subscriber station.